1. Field of the Invention
The present invention relates to a method and a device for imaging using several exposure times and a plurality of image sensor elements, such as, for example, those of an integrated CMOS semiconductor image sensor. In particular, the present invention relates to methods and devices for imaging, enabling a temperature and drift invariant adjustment of the output characteristic curves of an image sensor and, in particular, of an integrated CMOS semiconductor image sensor.
2. Description of Prior Art
For a picture record or picture detection, digital image detecting systems, in particular those using integrated CMOS semiconductor image sensors, are becoming increasingly important. In such image sensors, depending on an illumination, i.e. an incident light radiation, charge carriers are generated in a photodiode, a phototransistor or a photogate. Usually a charged capacitor is discharged by the charge carriers produced so that the discharge of the capacitor is a measure of the incident light radiation. In order to produce a signal indicating the incident light radiation or illumination the image sensor element is exposed with a predetermined exposure time, whereupon the voltage of the capacitor is sampled for reading out the image sensor element, i.e. for producing an output signal value. If there is no illumination, the sensor output signal sampled is at a maximum, while the sensor output signal sampled decreases with an increasing discharge of the capacitor and finally reaches an area of saturation. The usage of different integration times represents different amplifications so that for longer exposure times the area of saturation is already obtained at a lower sensor input signal. When short exposure times are used, the sensitivity for small sensor input signals is, however, too low.
In order to be able to obtain a greater control area, it is known in existing image acquisition systems to use different exposure times. Thus, the images obtained at the different exposure times are linked using calibration methods. For this, the quasi-linear sensor characteristic curve is passed slowly after manufacturing the sensor, i.e. before delivering to the customer, by a homogenous light source. The sensor output signals are mapped to a value by means of a look-up table, the value corresponding to a linear characteristic curve for the respective exposure time. These values are then stored in a non-volatile memory. A decisive disadvantage of this method is that characteristic curve changes occurring due to operating temperature variations and a long-time drift cannot be detected. Depending on the calibration algorithm, they can, however, deteriorate the system performance negatively, which requires a removal of the camera system, i.e. of the image record system, which may be expensive, and a recalibration. Thus, only the long-time drift can be compensated temporarily.
A further disadvantage of the image acquisition system described is that the memory requirement for storing the correction data is enormous. When calibrating, correction values relating to different output signal values of different pixels with no illumination, i.e. for an offset, and for amplification in order to map the nonlinear real characteristic curve to a linear characteristic curve must be calculated. For each possible combination of offset and amplification, the “real” output signals corresponding to the pixel output values must be stored in a look-up table. In a 1024×1024 pixel matrix having a pixel depth of 16 bits, two exposure times, i.e. amplifications, and a medium offset thus a memory requirement of four megabytes results for the look-up table. Previous realizations of highly dynamic camera systems having a multiexposure are thus only possible with considerable costs so that a practical realization of such systems has not been known.
U.S. Pat. No. 5,047,861 discloses a device for an image record with several image sensor elements, in which the output data of each image sensor element is corrected individually. For two correction values, a respective correction function is generated depending on the input image data. The correction data is stored in two allocation tables.
DE 198 16 003 A1 discloses a method for correcting the grey levels of images of a digital infrared camera. For each image sensor element of the camera, for each grey level, a correction coefficient is stored in an allocation table, wherein the image data is linearized pixel by pixel by the correction coefficient.
U.S. Pat. No. 5,420,421 relates to a method for compensating nonlinearities of an infrared image sensor having a plurality of sensor elements. Thus, for each image element, several exposure times are passed to obtain correction values. Intermediate values are produced by an interpolation.
DE 37 33 074 C2 is a circuit arrangement for a pixel-by-pixel correction of image signals of an image sensor consisting of several photo elements, in which the correction values are stored in a correction value memory. A group of characteristic curves applying to all the photo sensor elements is selected.